feat: add change_title MCP tool support to OpenCode

cli/src/modules/common/permission/BasePermissionHandler.ts

@@ -19,6 +19,7 @@ type AutoApprovalRuleSet = {
 const AUTO_APPROVE_TOOL_NAME_HINTS = [
     'change_title',
     'happy__change_title',
+    'hapi_change_title',  // OpenCode MCP tool pattern
     'geminireasoning',
     'codexreasoning',
     'think',

cli/src/opencode/opencodeLocalLauncher.ts

@@ -5,6 +5,9 @@ import { Future } from '@/utils/future';
 import { getLocalLaunchExitReason } from '@/agent/localLaunchPolicy';
 import { ensureOpencodeHookPlugin } from './utils/hookPlugin';
 import { buildOpencodeEnv } from './utils/config';
+import { ensureOpencodeConfig } from './utils/opencodeConfig';
+import { TITLE_INSTRUCTION } from './utils/systemPrompt';
+import { buildHapiMcpBridge } from '@/codex/utils/buildHapiMcpBridge';
 import type { OpencodeHookEvent } from './types';
 import type { OpencodeHookServer } from './utils/startOpencodeHookServer';
 import { createOpencodeStorageScanner, type OpencodeStorageScannerHandle } from './utils/opencodeStorageScanner';
@@ -263,6 +266,21 @@ export async function opencodeLocalLauncher(
     const opencodeConfigDir = resolveOpencodeConfigDir(session);
     ensureOpencodeHookPlugin(opencodeConfigDir, hookUrl, opts.hookServer.token);
 
+    // Start the hapi MCP server for change_title support (optional feature)
+    let happyServer: { url: string; stop: () => void } | null = null;
+    let opencodeConfigPath: string | null = null;
+    try {
+        const bridge = await buildHapiMcpBridge(session.client);
+        happyServer = bridge.server;
+        logger.debug(`[opencode-local]: Started hapi MCP server at ${happyServer.url}`);
+
+        // Generate opencode.json config with MCP server and instructions
+        const { configPath } = ensureOpencodeConfig(opencodeConfigDir, bridge.mcpServers.hapi, TITLE_INSTRUCTION);
+        opencodeConfigPath = configPath;
+    } catch (error) {
+        logger.debug('[opencode-local]: Failed to start hapi MCP server (change_title will be unavailable)', error);
+    }
+
     let storageScanner: OpencodeStorageScannerHandle | null = null;
     const messageRoles = new Map<string, string>();
     const sentTextParts = new Set<string>();
@@ -617,6 +635,10 @@ export async function opencodeLocalLauncher(
                 if (!env.OPENCODE_CONFIG_DIR) {
                     env.OPENCODE_CONFIG_DIR = opencodeConfigDir;
                 }
+                // Set OPENCODE_CONFIG to point to our generated config file (if MCP server started)
+                if (!env.OPENCODE_CONFIG && opencodeConfigPath) {
+                    env.OPENCODE_CONFIG = opencodeConfigPath;
+                }
 
                 await opencodeLocal({
                     path: session.path,
@@ -659,6 +681,10 @@ export async function opencodeLocalLauncher(
         if (storageScanner) {
             await storageScanner.cleanup();
         }
+        if (happyServer) {
+            happyServer.stop();
+            logger.debug('[opencode-local]: Stopped hapi MCP server');
+        }
     }
 
     return exitReason || 'exit';

cli/src/opencode/opencodeRemoteLauncher.ts

@@ -9,6 +9,7 @@ import type { OpencodeSession } from './session';
 import type { PermissionMode } from './types';
 import { createOpencodeBackend } from './utils/opencodeBackend';
 import { OpencodePermissionHandler } from './utils/permissionHandler';
+import { TITLE_INSTRUCTION } from './utils/systemPrompt';
 
 class OpencodeRemoteLauncher extends RemoteLauncherBase {
     private readonly session: OpencodeSession;
@@ -17,6 +18,7 @@ class OpencodeRemoteLauncher extends RemoteLauncherBase {
     private happyServer: { stop: () => void } | null = null;
     private abortController = new AbortController();
     private displayPermissionMode: PermissionMode | null = null;
+    private instructionsSent = false;
 
     constructor(session: OpencodeSession) {
         super(process.env.DEBUG ? session.logPath : undefined);
@@ -112,9 +114,16 @@ class OpencodeRemoteLauncher extends RemoteLauncherBase {
             this.applyDisplayMode(batch.mode.permissionMode);
             messageBuffer.addMessage(batch.message, 'user');
 
+            // Inject title instructions on first prompt
+            let messageText = batch.message;
+            if (!this.instructionsSent) {
+                messageText = `${TITLE_INSTRUCTION}\n\n${batch.message}`;
+                this.instructionsSent = true;
+            }
+
             const promptContent: PromptContent[] = [{
                 type: 'text',
-                text: batch.message
+                text: messageText
             }];
 
             session.onThinkingChange(true);

cli/src/opencode/utils/opencodeConfig.ts

@@ -0,0 +1,81 @@
+/**
+ * OpenCode configuration file generator.
+ *
+ * Generates opencode.json with MCP server configuration and instructions
+ * for the hapi change_title tool.
+ */
+
+import { mkdirSync, readFileSync, writeFileSync } from 'node:fs';
+import { join } from 'node:path';
+
+const CONFIG_FILENAME = 'opencode.json';
+const INSTRUCTIONS_FILENAME = 'hapi-instructions.md';
+
+interface McpServerEntry {
+    command: string;
+    args: string[];
+}
+
+interface OpencodeConfig {
+    $schema: string;
+    mcp: Record<string, {
+        type: string;
+        command: string[];
+        enabled: boolean;
+    }>;
+    instructions: string[];
+}
+
+/**
+ * Ensures the opencode.json config file exists with MCP server and instructions.
+ *
+ * @param rootPath - The OPENCODE_CONFIG_DIR path
+ * @param mcpServer - The hapi MCP server command configuration
+ * @param instructions - The instruction text to write to the instructions file
+ */
+export function ensureOpencodeConfig(
+    rootPath: string,
+    mcpServer: McpServerEntry,
+    instructions: string
+): { configPath: string; instructionsPath: string } {
+    mkdirSync(rootPath, { recursive: true });
+
+    // Write instructions file
+    const instructionsPath = join(rootPath, INSTRUCTIONS_FILENAME);
+    writeFileSafe(instructionsPath, instructions);
+
+    // Build opencode.json config
+    // Use absolute path for instructions since OpenCode resolves paths relative to project root
+    const config: OpencodeConfig = {
+        $schema: 'https://opencode.ai/config.json',
+        mcp: {
+            hapi: {
+                type: 'local',
+                command: [mcpServer.command, ...mcpServer.args],
+                enabled: true
+            }
+        },
+        instructions: [instructionsPath]
+    };
+
+    const configPath = join(rootPath, CONFIG_FILENAME);
+    const configJson = JSON.stringify(config, null, 2);
+    writeFileSafe(configPath, configJson);
+
+    return { configPath, instructionsPath };
+}
+
+/**
+ * Write file only if content has changed.
+ */
+function writeFileSafe(filePath: string, content: string): void {
+    try {
+        const current = readFileSync(filePath, 'utf-8');
+        if (current === content) {
+            return;
+        }
+    } catch {
+        // Ignore missing or unreadable file
+    }
+    writeFileSync(filePath, content, 'utf-8');
+}

cli/src/opencode/utils/systemPrompt.ts

@@ -0,0 +1,20 @@
+/**
+ * OpenCode-specific system prompt for change_title tool.
+ *
+ * OpenCode exposes MCP tools with the naming pattern: <server-name>_<tool-name>
+ * The hapi MCP server exposes `change_title`, so it's called as `hapi_change_title`.
+ */
+
+import { trimIdent } from '@/utils/trimIdent';
+
+/**
+ * Title instruction for OpenCode to call the hapi MCP tool.
+ */
+export const TITLE_INSTRUCTION = trimIdent(`
+    ALWAYS when you start a new chat - you must call the tool "hapi_change_title" to set a chat title. When you think chat title is not relevant anymore - call the tool again to change it. When chat name is too generic and you have a chance to make it more specific - call the tool again to change it. This title is needed to easily find the chat in the future. Help human.
+`);
+
+/**
+ * The system prompt to inject for OpenCode sessions.
+ */
+export const opencodeSystemPrompt = TITLE_INSTRUCTION;